Though numerous embodiments of framed hollow fiber membranes (referred to as "fibers" for brevity) have been disclosed in this art, and much effort has been expended to provide a "wafer" or "cell" which can be assembled to form a module, the effort has not resulted in a cost-effective module which is reliable, rugged and has wide commercial applications. Prior art cells have sacrificed good exposure of the fibers for packing density by making wafers with bundles of fibers which are in contact with one another to a greater or lesser degree in some wafers compared to others; and, have made wafers with fluid passages in the sides of their frames.
This invention is specifically directed to a cartridge formed by repetitively adding repeating units, each unit consisting essentially of a unitary frame of arbitrary shape and an array of fibers; and, to a module containing a stack of cartridges. The term "repeating unit" is used because such a unit, per se, is difficult to handle and has little practical utility. Such a repeating unit is referred to as a "unitary wafer" to provide better visualization of the physical form of a repeating unit. We know of no prior art cartridge which has been constructed with such "unitary wafers".
A cartridge of unitary wafers was never successfully constructed prior to this time for a number of inter-related reasons, one of which was the high risk of not making a leak-proof cartridge; another was a preoccupation with the ability to replace a defective cell when necessary. Not the least of reasons was that it was not evident how a reliable and economical cartridge could be constructed with simplicity. Moreover, there is no intimation in the prior art of the far-reaching benefits of constructing a cartridge directly, without first constructing a single cell.
Conceptually, the effort in the art has been directed towards providing a single cell, then coaxially assembling plural cells, because it was logical to construct a single cell. A single cell can be handled and checked before being assembled in a module. But a different approach, namely, a concept directed to constructing a reliable and rugged cartridge without making individual cells, led to a solution of the problems in the prior art, namely providing a module with high efficiency and reliability at an affordable cost.
For example, Nichols in U.S. Pat. No. 4,959,152, states "Hollow fiber membranes may be conveniently mounted in annular or similar frames or retainers having a continuous perimeter and an open central portion. The fibers are strung across the open central portion of the frame and the ends are embedded in the retainer thereby forming a wafer. The ends of the fibers are exposed at the outside surface of the retainer, giving access to the interior of the fibers, while the outside surfaces of the fibers are accessible in the open central portion of the retainer." (see col 1, lines 57-66). Soon thereafter he states "Tight sealing of adjacent wafers is essential to avoid contamination of retentate and permeate." Though not explicitly stated, Nichols recognized the importance of sealing plural layers of fibers in each wafer effectively, because he constructed a device to centrifuge an epoxy resin of appropriately chosen viscosity and quick-setting characteristics, to generate a potting ring through which the ends of the fibers protrude to discharge fluid flowing through the lumens of the fibers.
The effectiveness of the centrifugal force however was not restricted to ejecting the epoxy resin radially outward to be deposited against the inner periphery of his mold; the centrifugal force also displaced the fibers in each layer resulting in uncontrolled spacing of fibers and "gaps" which invite channeling. To counter such displacement, individual fibers running parallel to each other in the weft direction in each layer, were woven together ("tied") with warp filaments to form a flat sheet; or, each layer of fibers was adhesively secured to a contiguous layer with a suitable adhesive-coated filament placed on the upper and lower surfaces, respectively, or both, of each layer (see col 4, lines 56-66). Tying fibers together results in chafing at the "ties" and premature rupture of the chafed fiber; and, in entrapment of solids in a "cage" formed by an axial zone between tied fibers.
By "axial" we refer to the central axis of a module, along which axis a multiplicity of arrays of fibers are assembled. For convenience and ease of reference in most of the drawings, the central axis is referred to as being in the vertical direction unless otherwise stated. The arrays of fibers are referred to as being in the lateral plane, one axis of which is referred to as horizontal, and a direction at an angle to the horizontal in the lateral plane is referred to as being transverse.
Our goal was to construct an assembly of coaxially aligned wafers in fluid-tight relationship near the peripheries of their frames, each wafer carrying but a single row ("monolayer") of parallel spaced apart fibers without tying them together or interconnecting them, or potting their ends. Yet we wished to secure the monolayer of fibers in fluid-tight spaced-apart relationship near their terminal portions, in opposed sides (or opposed portions of the border) of a frame having a central or off-set through-passage ("feed channel") for carrying a feedstream to be treated. The problem of confining those terminal portions had little in common with the problem so recently solved by Nichols, namely of securing multiple layers by forming inner and outer potting rings of centrifuged resin, and removing the outer one.
More than a score of years earlier, Strand in U.S. Pat. No. 3,342,729 had to use a mesh of fibers which he sandwiched between two extruded or cast frame members, formed from a suitable thermoplastic polymeric material. The reason he was forced to use a mesh was because such a configuration of meshed fibers had inherent stability. A multiplicity of individual, loose fibers do not have such stability. The stability afforded by the mesh is sufficient to allow the fibers (as a mesh) to be handled and positioned between the frame members. Strand did not suggest positioning individual fibers, in side-by-side relationship between the frame members nor could he have done so without envisioning the possibility of providing an essentially planar array of individual spaced-apart fibers between frame members. The fibers as a mesh, sandwiched between two frame members, is referred to as a "cell" in Strand's invention.
Strand suggested making a cell as follows: "A mesh membrane can be sandwiched between two such (frame) members and the assembly subjected to heat sealing conditions whereby a unitary, integral cell member is provided. This means has the added feature of readily and securely bonding the members into an intimate joined relationship, but additionally avoids the need for any adhesive and sealant material and the attendant setting or drying time. Means can also be provided simultaneously to heat-seal the ends of any fibers protruding beyond the outer edge of the joinder of the two frames by causing the material of the frame to flow over the joinder forming a smooth surfaced seamless edge. Care must be exercised that the hollow fibers are not materially altered in any portion where flow therethrough is desired. The frames can be made in pairs with mating male and female fittings such as lugs and indents to facilitate and assure alignment of the various matching openings. Rapid production of the cells can be achieved by the foregoing means." (see col 7, lines 12-30).
Strand's only description of the fabrication technique he used to form a cell, required a laminated frame constructed from two laminar portions, each having congruent elongated through-passages in each side. The upper surfaces of first laminar portions and the lower surfaces of second laminar portions within the periphery defined by the through-passages in the sides, are coated with adhesive such as an epoxy resin, and the mesh (which is cut slightly larger than the frame) is sandwiched in the adhesive between the upper and lower laminar portions until the mesh is securely and permanently bound to, and held between the laminar portions.
As did others before and after Strand, he suggested a "great number of membrane cells be stacked one above another with very small separations between them (and in some instances one upon another) to develop an extremely large transfer area . . . " (see col 4, lines 25-28). The function of the through-passages, or elongated perforations, in the sides of the frame was to provide multiple flow-channels for permeate, that is, the sides were "functionally perforate". In operation, channels for permeate must be sealed in fluid-tight relationship with the central through-passage ("feed channel") of the frame. Therefore, it was essential that there be no leakage of fluid through openings or channels in a "sealed zone" of any frame. The sealed zone of a frame is defined in the '729 patent as the annular zone adjacent a conduit formed by axially aligned multiple through-passages in an assembly of frames. The sealed zone includes (i) intra-frame space between a fiber and the frame in which it is held, as well as (ii) inter-frame space between successive frame members. The intra-frame space between the outer wall of the fiber and the frame (in which it is held) is sealed by filling it with adhesive which sets over the fibers. The inter-frame space in Strand's module may be sealed with a gasket member, particularly if the assembly, referred to herein as a "stack" of frames, is to be disassembled. The inter-frame space may be sealed with adhesive to non-displaceably secure each frame to a successive frame when there is no intention to disassemble the frames.
The problem we addressed was quite different from the one addressed by Strand. We sought to form a "cartridge" which could be "ganged" with other cartridges to form a "stack" in a module, using appropriate gasket means for disassembly of the cartridges, if desired. Preferably, neither the fibers in each array, nor those of adjacent arrays were to be touching when a cartridge was assembled, and their spaced-apart relationship was to be maintained, except during operation under near-extreme conditions, by essentially only the thickness of the frames in which the fibers were held. Each wafer was to consist essentially of an array of substantially coplanar, non-displaceable, individual, essentially linear fibers, supported in a preferably substantially coplanar unitary laminar frame having a continuous periphery (that is, no separation of the frame). There were to be no flow channels in the sides of the frame, that is, each frame was to be "functionally imperforate". Such perforations as might be provided within the borders of a frame would be solely to position or mount the frame within a shell of a module.
We sought to avoid tying fibers to one another intermediate their terminal portions, either to adjacent fibers in a specific array, or to adjacent fibers in an array above or below the specific array. Fibers in contact with each other not only decrease the effective area of a module of multiple wafers, but exhibit a proclivity to chafe against each other, thus damaging the walls of contacting fibers. Still further, we sought to avoid potting the ends of the fibers, and machining one centrifuged (outer) layer of resin in which the ends of the fibers are plugged, to expose another centrifuged (inner) layer of resin in which the ends of the fibers are not plugged, as in the Nichols' cell.
The significance and importance of securing loose, individual linear fibers in an array of coplanar fibers having generally parallel longitudinal axes, is better appreciated by referring to numerous prior art cells in which fibers are looped about a frame before their ends are secured by potting them. Even before Strand's invention teaching opposed headers in the periphery of each cell, Lewis et al in U.S. Pat. No. 3,198,335 taught a cell in which fibers were also secured in a "header" of the cell, in loops or "hanks", rather than individually, and at least one end of each loop was secured by being potted in resin to form the header. (see col 6, lines 16-30). The desirability of using loops in a cell construction having a header built into the cell was reiterated and refurbished twenty years later in an improvement by Ostertag in U.S. Pat. No. 4,440,641. In the construction of such cells, the fibers must be looped because there is no other means for holding them in place before they are potted. In the wafer we sought to construct we sought to provide no header, the open ends of fibers being so disposed as to communicate with an annular space outside the wafer.
In addition to coping with the problem of positioning a large number of fibers precisely before they are potted, there are numerous pitfalls in "potting" the terminal portions of fibers in a fluid resin which is to be solidified. To begin with, one must find a resin which is sufficiently compatible with the fibers as to form a fluid-tight bond which will survive over the useful life of the module. After having found such a resin one must make sure that movement of the fibers near the resin does not damage the fibers due to the shearing action of the solid resin on their terminal portions, particularly if the pressure differential to be used in the module is substantial. Further, cutting and dressing the solid resin to expose the ends of the fibers may result in plugging many of the fibers, and is to be avoided.
As if these problems were not enough, one must cope with the geometry of the frame which is to support each array of fibers, whatever the configuration of the array, in any assembly of arrays to be housed in a module.
Since we decided to forego potting the fibers we had to find a fast and effective technique for securing the terminal portions of the fibers. This required development of a technique for securing the terminal portions upon or within the border of the frame in such a manner as both, to position the fibers before they are bonded to the frame, and also to provide adequate support at the terminal portions so as to maintain the fibers in spaced-apart relationship relative to other fibers in the assembly.
The development of this technique resulted in the ability to make a cartridge of wafers in which all frame members are permanently sealed, successively, one to another.
In a "cartridge" which is constructed by sequentially assembling frames and monolayer arrays, the number of arrays "n" held between successive frames is one less than the number of frames in which the fibers are held, since a cartridge must begin and end with a frame, that is, there are "n +1" frames; and n represents an integer 2 or greater, preferably in the range from 6 to 100.
It will be appreciated that, though the description of the invention herein is for "outside-in" flow of feed, the module containing a cartridge, or a stack of cartridges, may be equally well adapted for "inside-out" flow, for process considerations demanding such flow. By "outside-in" flow we refer to a feed flowing through the central feed channel in a cartridge or stack, so as to allow a portion of the feed to be separated as permeate. The portion separated as permeate flows from outside the walls of the membranes into their lumens. "Inside-out" flow is when the feed flows through the lumens of the fibers and the permeate is collected outside the membranes.
The cartridge, a module containing the cartridge or stack of cartridges, the method of constructing the cartridge, and the effectiveness of each of the foregoing in a variety of permeation processes, address the deficiencies of the prior art.